Analysis of rock pore space saturation distribution with Nuclear Magnetic Resonance (NMR) method. Part II

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Jadwiga Zalewska, Dariusz Cebulski Oil and Gas Institute, Krakow

Analysis of rock pore space saturation distribution

with Nuclear Magnetic Resonance (NMR) method.

Part II

Introduction Measurements accomplished with Nuclear Magnetic Resonance (NMR) method, carried out on fluid saturated rock samples, provide useful information to characterize their pore structure. The examinations reflect general sen-sitivity of NMR measurements on pore size distribution, which is of particular significance in applications used in petroleum geology [4, 5, 8].

Each pore size within the rock pore system has char-acteristic value of T2 transverse relaxation time and signal amplitude proportional to the volume of fluid contained in pores of that size. Low value of T2 transverse relaxation time is expected for small pores, while high value of the time values corresponds to large pores [7].

Routine NMR application used in Well Geophysics Department of INiG which enables evaluation of reservoir properties of rocks includes total and effective porosity and irreducible water saturation coefficient. Total porosity (KpNMR) is calculated from the whole T2 spectrum range, while effective porosity (KpNMR_ef) is calculated from NMR

signal of T2 time decays higher than 3 ms. Total NMR porosity is divided into three main constituents: poros-ity of free water pore space, capillary bound water and irreducible water. Porosity of free water pore space (Kp3) corresponds to long transverse relaxation times T2. Short T2 times are attributed to irreducible water (Kp1) bound in clay minerals or micropores [1]. According to Coates [3], irreducible water may be defined as volume of water that cannot relocate and cannot be displaced from pore space. Determination of free water volume (that freely moves), as well as bound water, enables determination of potential reservoir rock ability to deliver hydrocarbons in the process of reservoir production.

The aim of the study was evaluation of pore space saturation distribution for Upper Rotliegend Aeolian sandstones originating from G1, O-3 and R-1, R-2 and R-3 boreholes, for the same samples for which microto-mographic analysis was carried out, presented in Zalewska et al. part I [10].

Results of examinations Examinations with Nuclear Magnetic Resonance

meth-od were carried out with Maran-7 spectrometer, while analysis of pore space saturation distribution for rock samples was accomplished according to methodology described in Ciechanowska et al. [2].

The material under examination was rock samples origi-nating from drill cores representing Rotliegend formations from G-1, O-3, R-1, R-2 and R-3 boreholes.

Collective list of change ranges and mean values of NMR determined parameters and permeabilities for in-dividual boreholes was presented in table 1 and figure 1. The samples were characterized with:

• total porosity coefficient KpNMR ∈ (2.59÷17.43%) with

mean value equal to 9.69%. Approximately 60% of all samples under examination featured porosity under 10%, 31% samples had porosities in range from 10

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Table 1. Collective list of change ranges and mean values of NMR determined parameters and permeabilities for analyzed boreholes

Parameter Range of parameter change Mean value Range of parameter change Mean value Range of parameter change Mean value

from to from to from to

G-1 well (n = 12) O-3 well (n = 12) R region (n = 13)

Irreduc. water – Kp1 [%] 2.10 4.43 3.21 2.17 3.61 2.88 2.53 4.76 3.60

Capillary water – Kp2 [%] 0.41 5.45 3.64 0.33 4.89 3.21 2.58 6.37 4.00

Free water – Kp3 [%] 0.07 9.50 3.09 0.09 8.79 3.77 0.27 7.62 2.38

Total porosity – KpNMR [%] 3.67 17.43 9.94 2.59 16.45 9.86 7.27 16.45 9.98

Effective poros. – Kpef [%] 0.48 14.28 6.73 0.42 13.68 6.98 2.94 12.80 6.38

Irr. wat. satur. – Swnr [%] 16.64 86.92 39.31 16.84 83.78 36.85 20.59 58.36 38.82

Permeability – Kprz [mD] 0.203 15.137 3.91 0.321 110.013 20.47 0.862 99.747 11.38

Fig. 1. Changes of parameters determined with NMR method used for individual wells

a) volume of pore space filled respectively with: irreducible water (Kp1), capillary water (Kp2), free water (Kp3),

b) coefficients of total (KpNMR) and effective (KpNMR_ef) porosities, c) coefficient of irreducible water saturation (Swnr) a)

b)

c)

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up to 15%, and 10% of samples exhibited KpNMR

coef-ficient over 15%,

• effective porosity coefficient KpNMR_ef ∈ (0.42÷14.28%),

at average equal to 6.45%. Approximately 80% of all examined samples featured porosity below 10%, and 21% samples showed KpNMR_ef exceeding 10%,

• Swnr irreducible water saturation coefficient variation

in range of 16.64÷86.92%, with mean value – 39.75%, • capillary water content Kp2 ∈ (0.33÷6.37%), with mean

value equal to 3.53%,

• free water content Kp3 ∈ (0.07÷9.50%), with mean value equal to 2.93%,

• formation factor ranging within 0.20÷110.01 mD, at mean value equal to 11.40 mD. The highest number, that is almost 80% of all examined samples featured

Kprz permeability below 10 mD and only 6% of them

in excess of 90 mD.

Comparison of results obtained for G-1, O-3 wells and R region enable to state that values of NMR method determined parameters in individual wells are characterized by very similar mean values, starting from total porosity, through effective porosity, to end with irreducible water saturation coefficient. The most prominent differentiating parameter is permeability coefficient which varies from 3.91 mD in G-1 well, through 11.38 mD (R region) up to 20.47 mD (O-3 well).

Permeability examinations were carried out in Geology and Geochemistry Department of Oil & Gas Institute by team led by G. Leśniak [6].

Nevertheless, majority of samples originating from G-1 well is characterized by very broad pore size spectrum as well as single- or bimodal shape of T2 transverse relaxation time distribution (Fig. 2). Samples of total porosity ranging from 3.67÷17.43% and effective porosity 0.48÷14.28%

predominate in tested rock material. The following samples showed the highest porosity: 10726 (KpNMR = 17.43%,

KpNMR_ef = 14.23%; Fig. 2a) and 10729 (KpNMR = 17.13%,

KpNMR_ef = 14.28%; Fig. 2a), for T2 time distributions were located within the scale of high T2 values, which corre-sponds to high size of pores. These samples belonged to A2 facies formations.

Samples of the lowest porosity, both total and effective, (10733 KpNMR = 3.74%, KpNMR_ef = 1.44%; sample 10734

KpNMR = 3.67%, KpNMR_ef = 0.48%; Fig. 2b) are samples

belonging to P2 facies. Distributions of relaxation time for these drill cores are located within low T2 values, which corresponds to small pore size. Three samples assigned to A4 facies formations (Fig. 2b) demonstrate intermediate values from indicated porosity range.

Change of parameters determined with NMR method for samples from G-1 well, with consideration given to separated facies, are presented in Table 2.

In G-1 well (Table 2), A2 facies stands out among the three analyzed facies (A2, A4, P2), which has the best parameters:

• the highest mean free water contents Kp3 (4.49%), • the highest mean value of effective porosity coefficient

KpNMR_ef (8.67%),

• the lowest mean value of irreducible water saturation coefficient Swnr (29.81%).

Similarly as before, samples from O-3 well show wide range of pore sizes and T2 transverse relaxation time dis-tribution (Fig. 3). In this well it can also be observed that Aeolian sandstones of A2 facies have long times of T2 trans-verse relaxation. Offset of T2 times towards lower value takes place in samples of A4 and A5 facies sandstones, which is related to pore size decrease. Further offset of T2 times towards yet lower values takes place in formations

Fig. 2. T2 transverse relaxation time distributions for samples from G-1 well

a) samples belonging to A2 facies formations, b) samples belonging to A4 facies formation (red) and samples belonging to P2 facies formation (yellow)

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of P2 sandy playa facies, which gives evidence of small size of the pores. Free water dominates in A2 facies forma-tions, with exclusion of 10738 sample (Fig. 3a). Whereas in sample 10741, representing A4 facies formation, low percentage of free water is observed (Kp3 = 0.5%) and significant increase of capillary (Kp2 = 2.42%) and ir-reducible water (Kp1 = 3.61%) percentage in relation to A2 facies (Fig. 3b). Even lower (as compared to A4 fa-cies) percentage of free water (Kp3 = 0.23%) (Fig. 3b) is observed in sample 10742, which is representative of A5 facies formations.

Sample 10746, representing P2 facies, has the worst parameters of all the analyzed samples.

In O-3 well (table 3), similarly as before, A2 facies stands out among the analyzed facies, which also has the best parameters:

• the highest content of free water Kp3 (5.41%),

• the highest value of effective porosity coefficient

KpNMR_ef (9.38%),

Region R was represented by three boreholes (R-1, R-2 and R-3), and maximum values of T2 transverse relaxation time distributions for it had decidedly lower amplitude values (2700) (Fig. 4) than T2 transverse relaxation times determined for samples originating from G-1 (5500) (Fig. 2) and O-3 (5000) (Fig. 3). Mean value of total porosity val-ue for R region, calculated from examination of 13 rock samples, equals to KpNMR = 9.98%, and effective porosity

KpNMR_ef = 6,38%. The highest value of these parameters

was found for samples from R-3 well (sample 10761

KpNMR = 16.45%, KpNMR_ef = 12.80%) and from R-1 well

(sample 10753 KpNMR = 13.33%, KpNMR_ef = 9.51%) (Tab. 1).

Both of the samples originated from A2 facies formations. Table 2. Summary of change ranges and mean parameters determined with NMR method,

and permeabilities for individual facies in G-1 well

G-1 well

A2 facies A4 facies P2 facies

Irreducible water – Kp1 [%] 2.10 3.66 3.14 3.28 4.43 3.67 2.30 3.19 2.75

Free water – Kp3 [%] 0.41 9.50 4.49 0.38 3.25 1.80 0.07 0.13 0.10

Effective porosity – Kpef [%] 3.34 14.28 8.67 4.74 7.71 6.06 0.48 1.44 0.96

Irreducible water saturation –

Swnr [%] 16.64 52.08 29.81 29.91 48.31 38.22 61.50 86.92 74.21

a) b)

Fig. 3. Distribution of T2 transverse relaxation time for samples from O-3 well

a) samples belonging to A2 facies formations, b) samples belonging to formations of A4 (10741) and A5 (10742) facies – red, while samples belonging to P1 facies formations (10745) and P2 (10746) are marked with yellow color

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a) b)

c)

Fig. 4. Distributions of T2 transverse relaxation time

for rock samples from R region

a) R-1 well, b) R-2 well, c) R-3 well

Table 3. Summary of change ranges and mean parameters determined with NMR method, and permeabilities for individual facies in O-3 well

Parameter

Range of parameter

change Mean

value

Range of parameter

change Range of parameter change

O-3 well

A2 facies A4 + A5 facies P1 + P2 facies

Irreducible water – Kp1 [%] 2.65 3.15 2.81 2.97 3.61 2.17 3.29

Capillary water – Kp2 [%] 3.46 4.89 3.96 1.94 2.42 0.33 2.15

Free water – Kp3 [%] 1.07 8.79 5.41 0.23 0.50 0.09 1.10

Total porosity – KpNMR [%] 7.95 16.45 12.19 5.14 6.53 2.59 6.54

Effective porosity – Kpef [%] 4.68 13.68 9.38 2.17 2.92 0.42 3.25

Irreducible water saturation – Swnr [%] 16.84 38.87 24.38 57.78 55.28 83.78 50.31

Permeability – Kprz [mD] 3.464 110.013 29.98 1.080 3.352 0.321 1.015

In R region (Tab. 4) represented by 3 wells, A2 facies shows the best parameters in R-2 well, because it has: • the highest free water content Kp3 (4.08%),

• the highest mean value of effective porosity coefficient

KpNMR_ef (9.22%),

Formations of A2 facies in the remaining wells (R-1 and R-3) have mean parameter values (Kp3, KpNMR_ef ) close to each

other, but they are lower than in R-2 well, while mean Swnr

value is higher. Neither P1 facies nor P2 facies representa-tives were present in samples originating from this region. The next grouping of measurement results obtained with NMR method was carried out with the use of concentration analysis, which enabled separation of three rock groups featuring similar parameters describing pore space. Rock samples showing good reservoir properties were counted among the first group, while rocks samples showing poor reservoir properties were counted among the third group.

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12 samples were rated among the first group rocks, which represented solely Aeolian dune sandstones of A2 facies (Fig. 5, navy blue). Within this group: three sample originated from G-1 well (10730, 10729 and

Table 4. Summary of change ranges and mean parameters determined with NMR method, and permeabilities for A2 facies within R region

A2 facies Aeolian dune sandstones

R-1 well R-2 well R-3 well

Free water – Kp3 [%] 1.09 4.77 2.03 3.39 4.76 4.08 0.27 7.62 2.90

Total porosity – KpNMR [%] 7.27 13.33 9.84 12.28 12.29 12.29 7.06 16.45 10.51

Effective porosity – Kpef [%] 3.76 9.51 6.04 8.69 9.76 9.22 2.94 12.8 6.86

Irreducible water saturation

– Swnr [%] 28.66 48.28 40.21 20.59 29.23 24.91 22.19 58.36 38.58

10726), five from O-3 well (10744, 10743, 10739, 10737 and 10736) and four from R region (10753 R-1, 10754 R-2, 10757 R-2 and 10761 R-3). Samples from this group show decidedly the best reservoir proper-Table 5. Statistical analysis of parameters determined with NMR method,

separated on the grounds of concentration analysis (in individual groups)

Parameter Range of parameter changes Mean parameter _value _deviationStandard

from to Group I Capillary water – Kp2 [%] 3.48 6.37 4.71 0.80 Free water – Kp3 [%] 3.39 9.50 6.49 1.94 Total porosity – KpNMR [%] 12.28 17.43 14.26 1.99 Effective porosity – Kpef [%] 8.69 14.28 11.19 1.99

Irreducible water saturation – Swnr [%] 16.64 29.23 21.79 4.05

Permeability – Kprz [mD] 1.37 110.01 29.36 38.70 Group II Capillary water – Kp2 [%] 2.76 4.46 3.89 0.45 Free water – Kp3 [%] 0.38 4.95 2.07 1.21 Total porosity – KpNMR [%] 6.52 11.25 9.33 1.33 Effective porosity – Kpef [%] 4.42 8.52 5.96 1.26

Permeability – Kprz [mD] 0.27 15.14 4.97 4.82 Group III Capillary water – Kp2 [%] 0.33 2.93 1.89 1.00 Free water – Kp3 [%] 0.07 1.10 0.43 0.36 Total porosity – KpNMR [%] 2.59 7.27 5.58 1.64 Effective porosity – Kpef [%] 0.42 3.76 2.32 1.26

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ties: KpNMR ∈ (12.28÷17.43%), KpNMR–avg = 14.26%;

KpNMR_ef ∈ (8.69÷14.28%), KpNMR_ef–avg = 11.19% and

filtra-tion ones Kprz ∈ (1.37÷110.01 mD), Kprz–avg = 29.36 mD. Free water content prevails in pore space of these samples, and its volume is the highest among all of the distinguished types Kp3 ∈ (3.39, 9.50%), Kp3–avg = 6.49%. Percentage of capillary water is also the highest as compared to the remaining groups: Kp2 ∈ (3.48÷6.37%), Kp2–avg = 4.71%. The value of irreducible water saturation coefficient falls within Swnr ∈ (16.64÷29.23%) range, and its mean value

Swnr–avg = 21.79% is the lowest one when compared to

the second and third group.

The second group was most numerous and represented by 15 samples (Fig. 5, red color). Six samples originated from G-1 well (three from each A2 and A4 facies), three samples represented O-3 well (all from A2 facies), and six

from R region (four from A2 facies and one from each of A4 and A2/A5 facies). This group has weaker reservoir and filtration properties than group I discussed earlier:

KpNMR ∈ (6.52÷11.25%), KpNMR–avg = 9.33%,

KpNMR_ef ∈ (4.42÷8.52%), KpNMR_ef–avg = 5.96%,

Kprz ∈ (0.27÷15.14 mD), Kprz–avg = 4.97 mD. Free wa-ter saturation coefficient Kp3 adopts values from 0.38 up to 4.95% with mean value of 2.07%. The content of capillary water is also slightly lower than in group I:

Kp2 ∈ (2.76÷4.46%), Kp2–avg = 3.89%. Mean value of ir-reducible water saturation coefficient increases as compared to group I, and falls within range Swnr ∈ (24.27÷48.31%),

at mean value Swnr–avg = 36.45%.

12 samples were counted within the third group. The group showed the highest diversity. Three samples origi-nated from G-1 well and each sample represented P2, P1

and A2 facies (10734, 10733 and 10724, re-spectively). Four sam-ples originated from O-3 well and represent-ed P2, P1, A5, and A4 facies (10746, 10745, 10742 and 10741, re-spectively). Within R region one sample from each well was count-ed within this group, namely: 10752 R-1 from A2, 10756 R-2 from A2/A5 and 10758 R-3 from A2 facies. Fig. 5. Model distribution T2 transverse relaxation time curves, belonging to individual groups

Group I (having good reservoir properties) – navy blue, group II (having weak reservoir properties) – red, group III (having poor reservoir properties) – green

Table 6. Comparison of change ranges and mean values of parameters determined with NMR method and permeabilities for A2 facies formations

Parameter Range of parameter changes Mean value Range of parameter changes Mean value Range of parameter changes Mean value

Aeolian dune sandstones of A2 facies

G-1 well (n = 7) O-3 well (n = 8) R region (n = 10)

Free water – Kp3 [%] 0.41 9.50 4.49 1.07 8.79 5.41 0.27 7.62 2.79

Effective porosity – Kpef [%] 3.34 14.28 8.67 4.68 13.68 9.38 2.94 12.80 7.00

Irreducible water saturation

– Swnr [%] 16.64 52.08 29.81 16.84 38.87 24.38 20.59 58.36 36.50

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This group features poor reservoir properties (Fig. 5, green color). Values of total and effective porosity coef-ficient and permeability are the lowest among all the sepa-rated groups: KpNMR ∈ (2.59÷7.27%), KpNMR–avg = 5.58,

KpNMR_ef ∈ (0.42÷3.76%); KpNMR_ef–avg = 2.32%;

Kprz ∈ (0.20÷3.35 mD), Kprz–avg = 1.41 mD. Irre-ducible water saturation coefficient definitely prevails:

Swnr ∈ (46.27÷86.92%), Swnr–avg = 61.82%, and also

free water percentage is scarce: Kp3 ∈ (0.07÷1.10),

Kp3–avg = 0.43%. High value of Swnr parameter in these

samples is related to small (almost null) possibility of saturating them with hydrocarbons.

Table 6 contains comparison of change ranges and mean values of parameters determined with NMR method and permeabilities for examined samples originating from core dune A2 facies formations.

Summary It has been found on the basis of examination results carried out with NMR method and parameters determined in analyzed boreholes that Aeolian dune sandstones of A2 facies drilled in O-3 borehole, featured the best reservoir properties.

Samples from G-1 well feature slightly worse parameters of A2 facies, determined with NMR method, where mean

value of effective porosity coefficient decreased, and mean value of irreducible water saturation coefficient increased.

Samples from R region featured the poorest parameters of A2 facies as compared to O-3 and G-1 wells, for which the mean value of total and effective porosity were the lowest, while the mean value of irreducible water satura-tion coefficient was the highest.

The article was sent to the Editorial Section on 9.03.2011. Accepted for printing on 29.06.2011.

Reviewer: prof. zw. dr hab. inż. Andrzej Kostecki References

[1] Allen D.F., Flaum C., Bedford J., Castelijns K., Fair-hurst D., Gubelin G., Heaton N., Minh C.C., Norville M.A., Seim M.R., Pritchard T., Ramamoorthy R.: Trends in NMR

logging. Oil field Review no. 3, vol. 12, p. 2–19, 2000.

[2] Ciechanowska M., Zalewska J.: Analysis of reservoir rock

properties with the use of nuclear magnetic resonance.

Nafta-Gaz no. 1, 2002.

[3] Coates G.R., Xiao R., Prammer M.G.: NMR Logging

Principles and Applications. Halliburton Energy Services,

Houston 1999.

[4] Dullien F.A.L.: Porous media: Fluid transport and pore

structure. Academic Press, New York 1992.

[5] Kenyon W.E., Day P., Straley C., Willemsen J.: Compact

and consistent representation of rock NMR data from permeability estimation. SPE, Proc. 61st_{Annual Technical}

Conference and Exhibition, 1986.

[6] Leśniak G. et al.: Determination of processes leading to

creation of anomalously low porosity and permeability

in Aeolian sandstones in Poznań-Kalisz-Konin region, as well as perspectives of “tight-gas” reservoir discoveries in this zone. Documentation of 395/ST order, INiG Archive,

2009.

[7] Moss A.K., Zacharapoulos A., De Freitas M.H.: Shale

Volume Estimates from NMR Core Data. SCA2003-66,

2003.

[8] Sen P.N., Straley C., Kenyon W.E., Whittingham M.S.:

Surface-to-volume ratio, charge density, nuclear magnetic relaxation and permeability in clay-bearing sandstones.

Geophysics, 55, no. 1, 61–69, 1990.

[9] Zalewska et al.: Evaluation of Upper Rotliegend sandstones

pore space in G-R region with the use of nuclear magnetic resonance and X-ray computed microtomography.

Docu-mentation of 454/SW order. INiG Archive, 2010. [10] Zalewska J., Kaczmarczyk J.: Analysis of internal pore

structure of rock samples based on X-ray computed mi-crotomography data (Part I). Nafta-Gaz no. 8, 2011.

MSc Eng. Jadwiga ZALEWSKA – geologist, Aca-demy of Mining and Metallurgy alumna. Head of Well Log Geophysics Department in Oil and Gas Institute – Krakow. Accomplishes research and development works in scope of laboratory measure-ments of drilling cores and muds for quantitative interpretation of well logs. Author of 132 published works.

MSc Dariusz CEBULSKI graduated from the Fa-culty of Geology, Geophysics and Environment Protection, University of Science and Technology in Krakow, Department of Geophysics. He is cur-rently working in the Well-logging Department in the Oil and Gas Institute in Cracow. He deals with investigation of the petrophysical properties of rocks.